The invention relates to a developing apparatus for use in an electrophotographic copying machine, and more particularly, to such apparatus which converts an electrostatic latent image which is formed as a result of imagewise exposure into a visual image by electrostatically depositing one component developer on the latent image through a non-contact developing process.
A number of developing techniques are available for electrophotographic copying machines, and include a cascade technique, magnetic brush technique, powder cloud technique, jumping technique, impression technique and else, to cite a few typical examples for dry type developing techniques. The developers used may comprise a one component system and a two component system. The cascade and the magnetic brush technique are used with a two component system while the powder cloud technique, the jumping technique and the impression technique are used with a one component system.
To summarize the variety of developing techniques, the cascade technique and the magnetic brush technique which are used with a two component system provide a number of advantages including the stability of the developing process, and are actually in use in most copying machines which are commercially available. However, they have certain disadvantages. Considering the magnetic brush technique by way of example, the developer used with this technique comprises a toner and a carrier, and any change in the proportion of mixture thereof results in an adverse influence upon the optical density of the resulting image.
Considering the developing technique which is used with a single component developer, the powder cloud technique and the impression technique involve a disadvantage that during the developing process, the toner may be deposited not only on an image area, but also on a non-image area of an electrostatic latent image which is formed on a latent image carrying member, resulting in a so-called background fogging which represents a degradation in image quality. The fogging is caused by the absence of a force which acts to detach any toner which may be held attracted to a non-image area by physical influences other than electrostatic attraction.
The jumping technique excels others in this respect. For example, as disclosed in Japanese Laid-Open Patent Application No. 43,037/1979, a multiple magnet assembly may be rotatably disposed inside a cylindrical developer carrier to grow and maintain a brush of developer around the periphery of the carrier, and a blade may be utilized to tip the brush to provide a control over the thickness of the developer layer so that such thickness may be maintained to a given value. The layer thickness is controlled such that a developer cannot be brought into contact with a non-image area of the latent image while the developer may be attracted to an image area of the latent image by the charge which is present in such area, thus achieving the developing process in a non-contact manner. However, such control over the layer thickness requires a high mechanical accuracy for various parts of the apparatus, and still is subject to the influences of humidity and temperature, resulting in the difficulty to provide a stable layer thickness.
Japanese Laid-Open Patent Application No. 119,142/1981 discloses a developing apparatus in which an alternating bias electric field is applied between a photosensitive drum, which represents a carrier for an electrostatic latent image, and a developer carrier, and in which the deposition of a developer onto a non-image area of the latent image is avoided by causing the bias field to alternate between a phase which allows the developer to be transferred onto the latent image and another phase which causes the transferred developer to be transferred in the opposite direction toward the developer carrier.
However, in this apparatus, the electrostatic latent image which is formed and maintained on the photosensitive drum is disturbed by the bias field though the degree of disturbance is reduced. While this does not cause any significant problem when producing a single copy per exposure, the latent image will be greatly degraded when producing multiple copies per exposure. The quality of the finally produced copy image will be degraded, producing the so-called background fogging in the non-image area.